Unio crassus

As part of newly discovered reproductive behaviour, this mollusc crawls up to the edge of the water, exposing its excurrent aperture, and then lets loose a stream of water. The fountain of water often contains glochidia, and it is suggested that this spurting behavior may facilitate dispersal of mussel larvae[3] (video).[4]

The thick shelled river mussel declined during the 20th century everywhere in Europe due to deteriorating water quality,[5] habitat fragmentation and host fish limitation.[6]

Its native distribution is Europe and Western Asia.[1][7]

• It is mentioned in annexes II and IV of Habitats Directive.
• Croatia
• Czech Republic - in Bohemia, in Moravia,[8] endangered (EN).[9][10] Its conservation status in 2004-06 was unfavourable (U2) according to a report for the European Commission in accordance with the Habitats Directive.[11]
• Finland - in southern Finland. Vulnerable.[12]
• France[13]
• Germany - critically endangered (vom Aussterben bedroht)[14] In Germany this bivalve has disappeared from 90% of its former range.
• Netherlands - locally extinct. In the Netherlands it has not been seen alive after 1968 and is most likely extinct in that country.[15][16]
• Poland - endangered[17]
• Slovakia - nearly threatened[18]
• Sweden - very rare[19]
• Denmark - very rare[20]

Wikimedia Commons has media related to Thick shelled river mussel.

• The Mussel Project Web Site
• Fauna Europea